Gas Fills and Gap Width in Multiple-Glazed Units
Another improvement to the thermal performance of insulating glazing units involves reducing the conductance of the air space between the layers. Originally, the space was filled with air or flushed with dry nitrogen just prior to sealing. In a sealed-glass insulating unit, air currents between the two panes of glazing carry heat to the top of the unit along the inner pane and settle down the outer pane into cold pools at the bottom. Filling the space with a less conductive, more viscous, or slow-moving gas minimizes the convection currents within the space, reducing conduction through the gas and the overall heat transfer between the interior and exterior.
The use of low-conductance gas fills is far less common in commercial glazing than it is in residential windows. This results from the fact that solar control technologies are more important in typical commercial buildings than techniques for reducing heat transfer by conduction. However, as higher performance facades are developed, gas fills may become more common in commercial building windows as well.
Multiple Panes and Suspended Plastic Films
By adding a second pane, the insulating value of the window glass alone is doubled (the U-factor is reduced by half). As expected, adding a third or fourth pane of glass further increases the insulating value of the window, but with diminishing effect.
Triple- and quadruple-glazed windows became commercially available in the 1980s as a response to the desire for more energy-efficient windows (Figure 3-10). As each additional pane of glass adds to the insulating value of the assembly, it also reduces the visible light transmission and the solar heat gain coefficient. Additional panes of glass increase the weight and thickness of the unit, which makes mounting and handling more difficult and transportation more expensive. Prototype windows using very thin layers of glass (0.5-1.0 mm) have been fabricated but are not in commercial production.
It is apparent that there are physical and economic limits to the number of glass panes that can be added to a window assembly. However, multiple-pane units are not limited to glass assemblies. One innovation is based on substituting an inner plastic film for the middle layer of glass. The light weight of plastic film is advantageous, and because it is very thin, does not increase the unit thickness. As with triple- or quadruple-glazed windows, windows using plastic films decrease the U-factor of the unit assembly by dividing the inner air space into multiple chambers.
Figure 3-11 illustrates a triple-glazed window with a very low heat flow rate (low U-factor) as well as low-solar-heat-gain properties. There are three glazing layers and one low-E coating with 1/4-inch krypton gas fill in the cavities, and low-conductance edge spacers. In this case, the middle glazing layer is a suspended plastic film. The low-E coatings can be applied to the glass or plastic. Figure 3-12 illustrates a window with four glazing layers (two glass panes and two suspended plastic films). The combination of multiple glass panes and plastic films with low-E coatings and gas fills achieves very low center-of-glass U-factors. In this example, both low-E coatings have low-solar-gain properties in order to reduce cooling loads. The combination of multiple glass and plastic film layers with low-E coatings and gas fills has been used to achieve center-of-glass U-factors as low as 0.08. The properties of low-E coatings and tints are discussed in the following sections.
Glass is available in a number of tints which absorb a portion of the solar heat and block daylight. Tinting changes the color of the window and can increase visual privacy. The primary uses for tinted glass are reducing glare from the bright outdoors and reducing the amount of solar energy transmitted through the glass.
Tinted glazings retain their transparency from the inside, although the brightness of the outward view is reduced and the color is changed. The most common colors are neutral gray, bronze, and blue-green, which do not greatly alter the perceived color of the view and tend to blend well with other architectural colors.
Traditional tinted glazing, bronze and gray, often force a trade-off between visible light and solar gain. There is a greater reduction in visible transmittance than in solar heat gain coefficient (Figure 3-14). This can decrease glare by reducing the apparent brightness of the glass surface, but it also diminishes the amount of daylight entering the room. For windows where daylighting is desirable, it may be more satisfactory to use a high-performance tint or coating along with other means of controlling glare. Tinted glazings can provide a measure of visual privacy during the day, when they reduce visibility from the outdoors. However, at night the effect is reversed and it is more difficult to see outdoors from the inside, especially if the tint is combined with a reflective coating.
To address the problem of reducing daylight with traditional tinted glazing, glass manufacturers have developed high-performance tinted glass that is sometimes referred to as spectrally selective (Figure 3-15). This glass preferentially transmits the daylight portion of the solar spectrum but absorbs the near-infrared part of sunlight. This is accomplished with special additives during the float glass process. Like other tinted glass, it is durable and can be used in both monolithic and multiple-glazed window applications.
Spectrally selective glazings have a light blue or light green tint and have higher visible transmittance values than traditional bronze- or gray-tinted glass, but have lower solar heat gain coefficients. Because they are absorptive, they are best used as the outside glazing in a double-glazed unit. They can also be combined with low-E coatings to enhance their performance further. High-performance tinted glazings provide a substantial improvement over conventional clear, bronze, and gray glass, and a modest improvement over the existing green and blue-green color-tinted glasses that already have some selectivity.
Tinted glazing is more common in commercial windows than in residential windows. In retrofit situations, when windows are not being replaced, tinted plastic film may be applied to the inside surface of the glazing. The applied tinted films provide some reduction in solar gain compared to clear glass but are not as effective as spectrally selective films or reflective glue-on films, and are not as durable as tinted glass.
When heat or light energy is absorbed by glass, it is either convected away by moving air or reradiated by the glass surface. The ability of a material to radiate energy is called its emissivity. All materials, including windows, emit (or radiate) heat in the form of long-wave, far-infrared energy depending on their temperature (see Chapter 2). This emission of radiant heat is one of the important components of heat transfer for a window. Thus reducing the window’s emittance can greatly improve its insulating properties.
The solar reflectance of low-E coatings can be manipulated to include specific parts of the visible and infrared spectrum. This is the origin of the term spectrally selective coatings, which selects specific portions of the energy spectrum, so that desirable wavelengths of energy are transmitted and others specifically reflected. A glazing material can then be designed to optimize energy flows for solar heating, daylighting, and cooling.
A glazing designed to minimize summer heat gains, but allow for some daylighting, would allow most visible light through, but would block all other portions of the solar spectrum, including ultraviolet and near-infrared radiation, as well as long-wave heat radiated from outside objects, such as pavement and adjacent buildings. These low-E coatings still maintain a low U-factor, but are designed to reflect the solar near-infrared radiation, thus reducing the total SHGC while providing high levels of daylight transmission (Figure 3-19).
Figure 3-20 illustrates a low-solar-gain low-E coating on a bronze-tinted, double-glazed unit. Figure 3-21 shows the same coating on spectrally selective tinted glass. Low-E coatings can be formulated to have a broad range of solar control characteristics while maintaining a low U-factor.
As the SHGC falls in single-pane tinted glazings, the daylight transmission (VT) drops even faster, and there are practical limits on how low the SHGC can be made using tints. If larger reductions are desired, a reflective coating can be used to lower the solar heat gain coefficient by increasing the surface reflectivity of the material. These coatings usually consist of thin metallic or metal oxide layers. The reflective coatings come in various metallic colors-silver, gold, bronze-and they can be applied to clear or tinted glazing. The solar heat gain coefficient can be reduced by varying degrees, depending on the thickness and reflectivity of the coating, and its location in the glazing system. Some reflective coatings are durable and can be applied to exposed surfaces; others must be protected in sealed insulating glass units.
Figure 3-17 illustrates a highly reflective coating placed over a bronze-tinted, double-glazed unit. The emittance of the coating creates modest changes in the U-factor.
As with tinted glazing, the visible transmittance of a reflective glazing usually declines more than the solar heat gain coefficient. Reflective glazings are usually used in commercial buildings for large windows, for hot climates, or for windows with substantial solar heat gains. Reflective glazing is also used by many architects because of its glare control and uniform, exterior appearance.